How to render ground reflections without rendering the ground?
Not an answer strictly... but...

Use a mirror modifier:

Works for Cycles and Eevee ; )
You can improve it for instance using UV parameters of the modifier.
If you shift its U to 1:

that can give a clue that you can use in the shader, to obtain this for instance:

to use it you can make a node group that you can plug into the alpha input of the principled BSDF (node group as you may have several materials).

The top line uses UV X coordinate (U) to look if under 1 (the mirrored part is over 1 because of the parameter in the modifier).
The bottom line uses Z coordinate to tune a progressive value from 1 to 0 as Z goes down.